The term “expandable”, as applied to an existing buried pipe or culvert, is used as a defined term of art throughout this disclosure. By “expandable”, this disclosure refers to culverts and pipes having an existing wavy or folded annular or circumferential profile, such that, responsive to a controlled radial force, the “waves” or “folds” will collapse or “smooth out”, allowing a limited expansion of the effective inside diameter of the pipe without intentionally rupturing the pipe. It is expected that many culverts or pipes falling within this definition will be metal, and will be corrugated or “accordion” in profile. However, the term is not limited to corrugated or accordion profiles on metal pipes or culverts.
Expandable culverts of interest in this disclosure primarily include buried pipes that carry, for example, water under roads and structures, usually to allow a stream to flow under a road or to carry runoff from the uphill side of a road to the downhill side. Utility piping and other infrastructure may also be carried within such culverts. Such culverts can be made from various materials, but are often made from corrugated metal because it provides flexibility and strength while remaining relatively light and inexpensive. Consequently, expandable metal pipe culverts have been widely used in road construction projects over the last 50 years.
The service life of an expandable metal culvert varies, depending on factors such as climate, maintenance, water flow, and the condition of the surrounding soil. However, this type of culvert came into widespread use in the 1950s, and many are now reaching the end of their useful life and need to be repaired or replaced (or refurbished) before they fail. Expandable metal culverts can fail in different ways. For example, rust and corrosion can cause the pipe to leak, or even to disintegrate and collapse. Leaks can lead to erosion around the pipe and the resulting lack of structural support can cause the pipe to break. Pipe failure can wash out roads and bridges and cause environmental damage to the waterways they drain into.
Culverts can be repaired, or refurbished, by building a new culvert or digging the existing pipe up and replacing it (“open cut” methods). But these methods can be costly and time-consuming. Further, open cut methods may impractical because of traffic volume (the road will likely have to be closed during open cut operations), terrain, or climate. However, culverts can sometimes be refurbished without digging them up. This process is referred to in the industry as trenchless replacement technology. In this method, a new pipe is attached to a tool that is pushed or pulled through the existing damaged pipe. The tool head intentionally breaks or splits the old pipe as it drags the new liner pipe in behind it (this technique is also called “pipe bursting”). These methods allow culverts to be replaced with minimal disruption to traffic flow on any roadway above the culvert and with less impact on the waterway the culvert drains into. However, it should be noted that such “pipe bursting” techniques are “destructive” to the host pipe (i.e., the old pipe being replaced), rendering the host pipe effectively useless to provide support or peripheral protection, for example, to a new liner pipe.
One example of the destructive “pipe bursting” technology in use today is disclosed in Unitracc publication “Hydraulic and Static Pipe Bursting”, Feb. 16, 2011, available as of the date of this disclosure at: http://www.unitracc.com/know-how/fachbuecher/rehabilitation-and-maintenance-of-drains-and-sewers/rehabilitation/replacement-en/replacement-by-the-trenchless-method-en/unmanned-techniques-en/pipe-bursting-en/hydraulic-and-static-pipe-bursting-en. According to this reference, a hydraulically expandable tool head shatters a surrounding existing brittle host pipe (typically clay or unreinforced concrete) as it is drawn down the length of the existing pipe. A replacement pipe follows close behind the tool head.
A further example of current trenchless technology is disclosed in U.S. Pat. No. 4,602,495 to Yarnell. Yarnell is a “non-destructive” alternative to destructive “pipe bursting” techniques such as disclosed in Unitracc, described above. Yarnell teaches an expandable tool head being drawn down an existing brittle host pipe in which “irregularities” have made it difficult, for example, to draw a new liner pipe through the pipe. Such “irregularities” include neighboring sections of existing pipe becoming misaligned and no longer coaxial, or soil pressure causing sections of the brittle pipe to fracture and partially collapse, constricting the original inner diameter of the pipe. A conical nose and expandable “leaf members” on the tool head temporarily remediate the “irregularities”, primarily by pushing the broken host pipe back against soil pressure, so that the effective original internal diameter of the host pipe can be temporarily restored. At that point, an inner liner pipe can be drawn through.
Current destructive trenchless methods for replacing or refurbishing culverts are inadequate for some kinds of host pipes. Existing cutting and bursting techniques have had limited success on host pipes made from expandable materials such as corrugated metal. The principle upon which current technology “bursts” pipe requires a conical front end of the tool head (or “cutting head”) to be dragged through the existing pipe, forcing the pipe over the body of the cutting head until it fractures or “bursts”. The outside diameter of the body of the cutting head is thus chosen to be larger than the inside diameter of the pipe, causing the pipe to rupture as the cutting head is dragged through. There is thus a force placed on the existing pipe by the cutting head that has both longitudinal and radial components. Problems arise when this technique is used on flexible and expandable pipes such as corrugated pipes. Rather than bursting or splitting corrugated pipes, conventional techniques often compress the pipe longitudinally, which can cause the pipe to fold up in front of the tool like an accordion. Not only does this accordion effect make the overall pipe replacement process slower and more expensive, it can potentially cause the tool to get stuck in the old pipe or block the path for the new pipe. An existing expandable pipe may become so badly “accordioned” that a section may require spot digging and removal in order to complete the overall replacement job.
Further, non-destructive pipe replacement techniques in the prior art (such as the Yarnell disclosure, described above) have been directed to temporarily restoring an ailing host pipe to as close its original condition as possible, so that an inner liner pipe can be installed. Because the host pipe is temporarily restored to its original condition (or close to original), the thickness of the liner pipe, once installed, inevitably reduces the operational diameter of the repaired pipe. In applications where pipe flow or capacity is important, such operational diameter reduction can become disadvantageous.